Alfalfa water use pinpointed in saline, shallow water tables of Imperial Valley

Although alfalfa is the dominant water user in the Imperial Valley and is planted on 40% of its irrigated acreage, data is incomplete regarding its water use under the most common growing conditions: moderately saline, clay soils with a relatively shallow, moderately saline water table. In a study from 1996 through 1998, we determined the seasonal variation in the alfalfa crop coefficient (K_c), based on measurements of applied water and soil-moisture depletion, and the contribution from the water table aquifer. Using chloride mass-balance methods, we estimated that the shallow water table contributed approximately 12% of the total applied water during the study, of which just over 8% occurred during the first year. Correcting the values for water table contributions (WTC) to crop water use removed the increasing trend in values during the study and resulted in an overall study crop coefficient of 0.84. The decrease in WTC was the result of a doubling in soil salinity to 12 deciSiemens per meter at the 3- to 4-feet-depth interval by the second year. For moderately saline growing conditions in the Imperial Valley, the alfalfa crop coefficient is smaller than elsewhere in the southwestern states, where values closer to one are common.